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Biomedical subjects

B N Cuffin

Publications and source records attributed to B N Cuffin.

At least 19 recordsLinked to original sources
PubMed2001

Experimental tests of EEG source localization accuracy in spherical head models.

OBJECTIVES: The locations of electrical sources in the brain can be calculated using EEG data. However, the accuracy of these calculations is not well known because it is usually not possible to compare calculated source locations with actual locations since little accurate location information is available about most sources in the brain. METHODS: In this study, sources at known locations are created by injecting current into electrodes implanted in the brains of human subjects. The locations of the implanted and scalp EEG electrodes are determined from CTs. The EEG signals produced by these dipolar sources are used to calculate source locations in spherical head models containing brain, skull, and scalp layers. The brain and scalp layers have the same electrical conductivity while 3 different skull conductivity ratios of 1/80th, 1/40th, and 1/20th of brain and scalp conductivity are used. Localization errors have been determined for 177 sources in 13 subjects. RESULTS: An average localization error of 10.6 (SD=5.5) mm for all 177 source was obtained for a skull conductivity ratio of 1/40. The average errors for the other ratios are only a few millimeters larger. The average localization error for 108 sources at superior locations in the brain is 9.2 (4.4) mm. The average error for 69 inferior location sources is 12.8 (6.2) mm. There are no significant differences in localization accuracy for deep and superficial sources. CONCLUSIONS: These results indicate that the best average localization that can be achieved using a spherical head model is approximately 10 mm. More realistic head models will be required for greater localization accuracy.

Algorithms↗
PubMed1999

Accuracy of EEG dipole source localization using implanted sources in the human brain.

OBJECTIVES: The location of electrical sources in the brain can be estimated by calculating inverse solutions in which the location, amplitude and orientation of the electrical sources are fitted to the scalp EEG. To assess localization accuracy of the moving dipole inverse solution algorithm (ISA), we studied two patients who had depth electrodes implanted for presurgical planning of epilepsy surgery. METHODS: Artificial dipoles were created by connecting a single sine wave pulse generator to different pairs of electrodes in multiple orientations and depths. Surface EEG recordings of the resulting pulses were evaluated with the ISA using a 4-shell spherical head model and plotted on the subjects' MRI. Dipole localization errors were evaluated with respect to the number of averaged pulses, different electrode montages and different dipole locations and orientations. RESULTS: Dipoles located at 40-57 mm from the scalp surface had localization errors that were greater than those located at 62-85 mm. Localization accuracy improved with increasing numbers of pulses and recording electrodes. Results with a standard 10-20 array of 21 electrodes showed an average localization error of 17 mm, whereas 41 electrodes improved this to 13 mm. Mean angular errors were 31 and 30 degrees, respectively. CONCLUSIONS: The ISA was able to differentiate between tangential and radial dipoles. We conclude that our implementation of the ISA is a useful and sound method for localizing electrical activity in the brain.

Brain↗
PubMed1998

Accuracy of electroencephalographic dipole localization of epileptiform activities associated with focal brain lesions.

We evaluated the accuracy of an electroencephalographic (EEG) localization technique (dipole inverse solution) in a consecutive series of 12 focal intracerebral lesions of diverse etiologies whose EEGs showed interictal spike activity or rhythmic activity at seizure onset. The calculated equivalent dipole was plotted on three axes in the patients' magnetic resonance image, and the distance between the dipole and the lesion margin was measured assuming that the shell of the lesion constituted an epileptogenic region. In all cases the dipole localized closer than 0.8 cm to the nearest lesion margin. In addition, we compared the postsurgical outcome of 6 patients to the dipole localization and the resection margins. In all 6 patients in whom the dipole, and hence the estimated seizure generator, was removed the surgical outcome was favorable. We conclude that the inverse solution algorithm is a promising method for using the scalp EEG to localize the sources of electrical activity in the human brain in routine clinical electroencephalography and provides three-dimensional data not available from conventional analysis.

Adult↗
PubMed1996

EEG localization accuracy improvements using realistically shaped head models.

A model of the head must be used in making estimates of the locations of electrical sources in the brain using electroencephalograms (EEG's) measured on the scalp. In part, the accuracy of these estimates is dependent on how accurately the model represents the actual head. In most work performed to date, spherical models of the head have been used. This paper presents results in which the estimates of source location are made in realistically shaped head models. Techniques for accurately and conveniently developing realistically shaped head models from CT's, MRI's, X rays, and/or physical measurements are also presented. Realistically shaped head models are developed for three subjects with electrical sources implanted at known locations in the brain. Localization accuracy is found to be significantly better in the realistically shaped head models than in spherical models if EEG's with good signal-to-noise ratio are used.

Computer Simulation↗
PubMed1995

A method for localizing EEG sources in realistic head models.

A computationally practical method for performing moving dipole calculations to localize EEG sources in realistic, boundary element (integral equation) type of head models is presented. This method makes use of a rapid method of solving the forward problem of calculating the EEG's produced by a dipole in a realistic head model. This rapid forward calculation method allows the use of standard Simplex search techniques to solve the inverse problem of localizing electrical sources in the brain from EEG's measured on the scalp.

Electroencephalography↗
PubMed1993

Electrical source analysis of auditory ERPs in medial temporal lobe amnestic syndrome.

Auditory event-related potential (ERP) components have been anatomically linked to temporal lobe structures and functionally related to attentional and memory processes. We recorded auditory ERPs using an "oddball" paradigm from two patients with amnestic syndromes secondary to medial temporal lobe encephalitic infections. The oddball paradigm elicits the exogenous N1 and P2 components, and the endogenous N2 and P3 components. Electrical source analysis was used to test for alterations in source strength and orientation in these patients compared to control subjects. Symmetric dipoles placed in the temporal lobe region were used to measure ERP component activity. In the patient with a lesion confined to the left, medial temporal lobe, including the posterior hippocampus, dipole orientation was displaced anteriorally. In the patient with lesions to the anterior medial temporal lobe, temporal poles, and orbital frontal cortex, the negative components of the ERP (N1 and N2) were reduced in the right hemisphere, accompanied by disturbed orientation. These findings are consistent with other evidence that the different components of the auditory ERP can be dissociated on the basis of lesion effects, and that the antero-posterior extent of encephalitic lesions may play an important role in modulating ERP abnormalities.

Acoustic Stimulation↗
PubMed1993

Effects of local variations in skull and scalp thickness on EEG's and MEG's.

Many studies have been performed on the effects of various features of head geometry on electroencephalograms (EEG's) and magnetoencephalograms (MEG's) and on the accuracy with which electrical sources in the brain can be localized using these measurements. However, to date no studies have been performed of the effects of local variations in skull and scalp thickness. This paper presents a computer modeling study of the effects of such local variations. The results obtained in this study indicate that local variations in skull and scalp thickness have effects on EEG's and MEG's which range from a simple intuitive effect to complex effects which depend on such factors as source depth and orientation, the geometry of the variation in skull and scalp thickness, etc. These results also indicate that local variations in skull and scalp thickness cause EEG localization errors which are generally much less than 1 cm and MEG localization errors which are even smaller. These results also indicate that multichannel and single-channel MEG measurements will produce localization errors of approximately the same amplitude when there is a bump on the external surface of the head but that multichannel measurements will produce significantly smaller localization errors than single-channel measurements when a depression is present in that surface.

Action Potentials↗
PubMed1991

Tests of EEG localization accuracy using implanted sources in the human brain.

The accuracy with which electrical sources in the brain can be localized by using electroencephalograms measured on the scalp is not well known. In this study, tests of localization accuracy were performed by using implanted dipolar sources in the human brain. These dipoles are created by passing a weak (subthreshold) current through intracerebral electrodes implanted in the brains of epileptic patients for seizure monitoring. The locations of these dipoles are accurately known from roentgenographs. First, 16 electroencephalograms produced by these dipoles were recorded, then inverse solutions were calculated for the apparent sources of these electroencephalograms. Finally, the locations of the apparent sources were compared with the known locations of the implanted dipoles to determine localization error. The average localization error for a total of 28 dipoles in 3 subjects was 1.1 cm. These results indicate that good localization accuracy for focal sources in the brain can be provided by scalp electroencephalograms.

Brain↗
PubMed1991

EEG versus MEG localization accuracy: theory and experiment.

We first review the theoretical and computer modelling studies concerning localization accuracy of EEG and MEG, both separately and together; the source is here a dipole. The results show that, of the three causes of localization errors, noise and head modelling errors have about the same effect on EEG and MEG localization accuracies, while the results for measurement placement errors are inconclusive. Thus, these results to date show no significant superiority of MEG over EEG localization accuracy. Secondly, we review the experimental findings, where there are again localization accuracy studies of EEG and MEG both separately and together. The most significant EEG-only study was due to dipoles implanted in the heads of patients, and produced an average localization error of 20 mm. Various MEG-only studies gave an average error of 2-3 mm in saline spheres and 4-8 mm in saline-filled skulls. In the one study where EEG and MEG localization were directly compared in the same actual head, again using dipoles implanted in patients, the average EEG and MEG errors of localization were 10 and 8 mm respectively. The MEG error was later confirmed by a similar (but MEG-only) experiment in another study, using a more elaborate MEG system. In summary, both theory and experiment suggests that the MEG offers no significant advantage over the EEG in the task of localizing a dipole source. The main use of the MEG, therefore, should be based on the proven feature that the MEG signal from a radial source is highly suppressed, allowing it to complement the EEG in selecting between competing source configurations.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain↗
PubMed1991

Moving dipole inverse solutions using MEGs measured on a plane over the head.

Since magnetoencephalograms (MEGs) are measurements of the magnetic field produced in the air around the head by electrical sources in the brain, it is possible to measure MEGs on some regular surface over the head such as a plane. Such measurements are easier to make than traditional measurements at a fixed distance from the head. This paper presents results of computer modeling studies of source localization errors caused by using MEGs measured on a plane over the head. It was found that non-spherical head shape does not have a greater effect on localization accuracy for measurements on a plane than for traditional fixed-distance measurements. The source localization errors were less than 1 cm for both types of measurement for sources in the cortical region of the brain. Localization errors were found to increase for sources at greater depth in the brain, but the errors using measurements on a plane were not found to be significantly larger than those using traditional measurements. Hence, because of the ease with which measurements on a plane can be made, more wide-spread use of such measurements should be considered.

Biophysical Phenomena↗
PubMed1991

Developing a more focal magnetic stimulator. Part I: Some basic principles.

Some general properties of currents induced by magnetic stimulators in volume conductors of any shape are first reviewed. Then, the property of focality (concentration at some internal point) of the current induced in a spherical model of the head is discussed, due to coils of various orientations and configurations. It is shown that one important property for focality is the complete absence of the radial component of current, regardless of the coil used. The theoretically computed current distributions in the sphere produced by three different coils are illustrated. These are (1) a circular coil parallel to the head (very nonfocal). (2) a circular coil erect on the head (more focal), and (3) a figure-eight coil parallel to the head (most focal). A computational search for yet more focal coil configurations shows that the figure-eight focality can only be altered somewhat, but basically not improved. A plot of focality of the figure-eight coil versus the diameter of its coils shows an improvement in focality with decreasing diameter down to one cm. At this diameter, the induced current at a level 2 cm below the coil is concentrated in a band 1.5 cm wide (half-maximum points). Finally, it is noted that as the diameter of the figure-eight coil is decreased, the current in the coil necessary for stimulation increases rapidly, with increasing engineering problems.

Animals↗
PubMed1991

Eccentric spheres models of the head.

Equations are derived for electric potentials (electroencephalograms) and magnetic fields (magnetoencephalograms) produced by dipolar sources in three eccentric spheres models of the head. In these models, I) the thickness of the layer representing the skull varies around the model, II) the thickness of the scalp layer varies, and III) the electrical conductivity of an eccentric spherical "bubble" in the brain region varies. Using these equations, it was found that variations in these features of the models have at most only small effects on the general spatial patterns of the electric potentials and the radial component of the magnetic fields. However, some significant effects on the amplitudes were found. The effects of the variations in the skull and scalp layer thicknesses on the field amplitudes were found to be significantly smaller than on the potential amplitudes. The effects on the field amplitudes of the variations in the bubble conductivity were found to be only somewhat smaller than on the potential amplitudes. It was also found that the effects of variations in these features of the models on source localization accuracy were significantly smaller for inverse solutions using fields than for solutions using potentials.

Electric Conductivity↗
PubMed1990

MEG versus EEG localization test using implanted sources in the human brain.

It is believed that the magnetoencephalogram (MEG) localizes an electrical source in the brain to within several millimeters and is therefore more accurate than electroencephalogram (EEG) localization, reported as 20 mm. To test this belief, the localization accuracy of the MEG and EEG were directly compared. The signal source was a dipole at a known location in the brain; this was made by passing a weak current pulse simulating a neural signal through depth electrodes already implanted in patients for seizure monitoring. First, MEGs and EEGs from this dipole were measured at 16 places on the head. Then, computations were performed on the MEG and EEG data separately to determine the apparent MEG and EEG source locations. Finally, these were compared with the actual source location to determine the MEG and EEG localization errors. Measurements were made of four dipoles in each of three patients. After MEGs with weak signals were discounted, the MEG average error of localization was found to be 8 mm, which was worse than expected. The average EEG error was 10 mm, which was better than expected. These results suggest that the MEG offers no significant advantage over the EEG in localizing a focal source. However, this does not diminish other uses of the MEG.

Adult↗
PubMed1990

Effects of head shape on EEG's and MEG's.

This paper presents results of computer modeling studies of the effects of head shape on electroencephalograms (EEG's) and magnetoencephalograms (MEG's) and on the localization of electrical sources in the brain using these measurements. The effects of general, nonspherical head shape on EEG's and MEG's are determined by comparisons of EEG and MEG maps from nonspherical head models with corresponding maps from a spherical head model. The effects on source localization accuracy are determined by calculating moving dipole inverse solutions in a spherical head model using EEG's and MEG's from the nonspherical models and comparing the solutions with the known sources. It was found that nonspherical head shape can produce significant changes in the maps produced by some sources in the cortical region of the brain. However, it was also found that such deviations of the head from sphericity produce localization errors of less than approximately 1 cm. No significant differences in the effects of such deviations on EEG's and MEG's were found. Finally, it was found that most such deviations do not cause a dipolar source which is perpendicular to the surface of the head model to produce a significant magnetic field; such a source produces zero magnetic field in a sphere.

Brain Mapping↗
PubMed1987

The role of model and computational experiments in the biomagnetic inverse problem.

This is a review of the role of model and computational experiments in studies of the part of the biomagnetic inverse problem that deals with the determination of electrical sources in the body using magnetic measurements around the body. Results from modelling studies of the forward problem that are important for the inverse problem are also reviewed. An evaluation is made of the adequacy of various models of the body for use in the biomagnetic inverse problem. This evaluation indicates that simple torso models, e.g. a semi-infinite volume or sphere, are probably inadequate. The review of the modelling studies of the inverse problem includes the effects of noise, source modelling errors, body modelling errors and measurement errors on the accuracy of source localisation methods using magnetic measurements. Source modelling errors are caused by differences between an actual complex source in the body and the simple model of it used in most source localisation methods; body modelling errors are caused by differences between the actual body and a simple model of it. The review indicates that typical experimental noise will only cause significant source localisation errors for inverse solutions calculated using fewer than approximately ten measurement points; it also indicates that source modelling errors must be rather large to be detectable when typical experimental noise is present. In addition, the review indicates that many experimental measurement errors will not cause significant localisation errors. The effects of body modelling errors are largely unknown. Suggestions for further biomagnetic inverse problem research are given. These include the development of more realistic models of the body, the experimental verification of such models and source localisation methods, and the development of methods for detecting and localising distributed or multiple discrete sources.

Electric Conductivity↗
PubMed1987

A method for combining MEG and EEG to determine the sources.

A three-step method is presented which combines an MEG and EEG map over the head to solve the inverse problem (to determine the sources). This method uses the feature that the MEG does not see a radial source, but only a tangential source, while the EEG sees both. A first test is also made of the method, using computer simulation, and the results presented. The purpose of the test is to see if the method is valid with noisy MEG and EEG data, and when some modelling errors are present; a single dipole source was used in a spherical head. It was found that the method works well when the RMS noise at each map location is 5% of the maximum MEG and EEG (readily attained in practice), but breaks down when the noise is 10% (quite noisy data). The modelling errors involved grid size, head radius and distance to the MEG coil, and were studied only through the first step of the method; with errors in a reasonable range, this limited test again worked well.

Brain↗
PubMed1985

Electrical sources in human somatosensory cortex: identification by combined magnetic and potential recordings.

Magnetic fields and electrical potentials produced by neuronal activity have different properties that can be used for the identification of electrical sources in the human brain. Fields and potentials occurring 20 to 30 milliseconds after median nerve stimulation in human subjects were compared in order to investigate the sources of evoked potential components that have been attributed by different investigators to the thalamus or thalamocortical afferents, to separate radial sources in somatosensory cortex and motor cortex, or to a tangential source in somatosensory cortex. The magnetic and potential wave forms were highly similar in morphology, and their spatial distributions were centered over sensorimotor cortex, were dipolar in shape, and differed in orientation by approximately 90 degrees; distances between the minimum and maximum of the magnetic distributions were about 60 percent of those of the potential distributions. These results cannot be accounted for by thalamic sources or radial cortical sources alone, but are consistent with a tangential source in somatosensory cortex, with an additional smaller contribution from radial sources.

Electrophysiology↗